This invention relates to corrosion inhibitors suitable for inhibiting the corrosion of aluminum and aluminum alloys, and to a method for protecting the surfaces of aluminum and aluminum alloys against attack by corrosive aqueous media. More particularly, the invention relates to improved solid corrosion inhibitor compositions comprising a caustic compound and at least one soluble corrosion inhibitor for use with corrodible devices for automatic addition of corrosion inhibitors to coolant systems. The invention is particularly useful in preventing corrosion in aluminum radiators used in the cooling systems of internal combustion engines.
Engine coolants for the cooling system of an automotive vehicle usually contain ethylene glycol and a small percentage of diethylene gylcol. This fluid is diluted with water to provide a 50% or lower concentration of glycol depending on the desired freezing point for the coolant system. Most companies that manufacture and/or distribute ethylene glycol for coolant systems add corrosion inhibitors to the solution to prevent corrosion of the copper-brass traditionally used in the manufacture of vechicle radiators.
These inhibitors usually are a mixture of one or more inorganic salts, such as phosphates, borates, nitrates, nitrites, silicates or arsenates, and an organic compound, such as benzothiazole, tolylthiazole or mercaptobenzothiazole, to prevent copper corrosion. The solution is generally buffered to a pH of 8 to 10 to reduce iron corrosion and to neutralize any glycolic acid formed in the oxidation of ethylene glycol. Most companies recommend a maximum of one or two years' service for their antifreeze coolant, however, it has been found that the average car owner does not follow the owner's instruction manual to maintain -20.degree. F. protection for the coolant system and does not check the coolant to determine if it is rusty or dirty. Many owners only add water when the antifreeze is lost through leakage or hose breakage. This is more likely to occur in the southern part of the country than in northern areas.
In normal passenger car service, 25% of the cars require coolant system servicing after only one year; after two years this percentage rises to 50%. With normal copper-brass radiators, and even more so with aluminum systems, it is extremely important that the antifreeze or coolant mixture contain 50 to 55% of the correctly inhibited ethylene glycol. A reduction to a mixture of 33% ethylene glycol--67% water will increase metal corrosion significantly. This is especially important with higher temperature coolant systems which are becoming more common with the increased use of emission controls.
Also, with the increasing emphasis on gas mileage of the new automobiles, cars are being downsized and reduced in weight through the substitution of lightweight metals or plastics for iron and steel where practical. In the automotive coolant systems, aluminum radiators are being utilized instead of the conventional copper-brass radiators previously used. As above noted, an aluminum radiator is more susceptible to the corrosive action of a coolant or antifreeze that is low in the percentage of ethylene glycol and/or where an insufficient amount of corrosion inhibitor is present in the coolant. In such a system, additional corrosion inhibitor must be added or the aluminum will begin to corrode by pitting at a rapid rate.
Devices for automatically adding corrosion inhibitor to the system are known in the art. One such system, disclosed in U.S. Pat. No. 4,333,516 provides a container having a corrodible portion and filled with corrosion inhibitor. As the coolant becomes corrosive, the container corrodes and is quickly penetrated. Because the corrodible portion of the container is considerably thinner than the material forming the radiator, penetration will occur well before significant attack on the radiator can occur. Once penetration is achieved, the coolant will dissolve or disperse the inhibitor into the coolant system and stop or retard further corrosion. Where solid inhibitors are employed, the rapidity with which further corrosion will be inhibited will depend in part upon the speed with which the solid inhibitor can be dissolved. To speed the dissolution of the inhibitor, resort may be had to means for destroying the corrosion-weakened portion of the container such as the use of internal springs or the use of compositions which expand upon hydration. This in turn greatly increases the exposure of the solid inhibitor to the coolant and hastens the dissolution thereof.
As further aid to rapid dispersion and dissolution, it is generally better practice to employ dry, granulated or powdered corrosion inhibitors. However, as a practical matter, it would be desirable to provide the solid corrosion inhibitor in a compressed or tightly compacted form in order to reduce the size of the container and to maximize the amount of corrosion inhibitor that can be included therein. Speeding the destruction of the corrodible portion of the container after penetration by coolant to enhance the rate of dissolution becomes very important where the corrosion inhibitor is a compacted or compressed solid.